Pump ED 101
AIR VS OIL FILLED
SEWAGE PUMP MOTORS
Joe Evans, Ph.D
http://www.pumped101.com
Why would anyone want to fill a perfectly good electric motor with oil? If
you to ask the manufacturer’s of oil filled submersible pump motors this
question, they will give a number of good reasons. On the other hand, the
manufacturer’s of air filled units will provide just as many good reasons why
one should not!
This short paper will explore this question in depth and investigate the pros
and cons of each design. I will also try to expose any myths that each group
of advocates may have inflicted upon the other. Although we may not decide
upon a single clear choice for all applications, we should gain a better
understanding of the capabilities of each.
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MOTOR ENCLOSURES
Let us start with a short review of non-submersible motors and then
compare their characteristics to those of submersible motors. I will also use
these comparisons as we debate the pros and cons of each design.
The vast majority of electric motors are not filled with oil. A couple of
notable exceptions include sealed refrigeration units and certain hydraulic
systems that use oil for bearing lubrication and cooling. These oil filled
applications are analogous to water filled circulating and submersible turbine
pump motors that also use water for bearing lubrication and cooling. The
rest, however, have nothing but air within the motor housing. They may use
oil as a bearing lubricant, as is the case with vertical hollow shaft motors,
but that oil is isolated from the stator housing.
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For a general introduction to electric motors check out the tutorials on single and three
phase motors at http://www.pumped101.com
The two most common enclosures found in NEMA frame motor construction
are the open drip proof (ODP) and totally enclosed fan cooled (TEFC)
designs. The ODP motor is open to atmosphere and uses an internal fan to
force outside air through the motor housing. Because outside air is always
moving through the motor housing, cooling is simple and efficient. There are
times, however, when contaminants within the air can damage the motor
windings and the ODP enclosure is not the best choice.
The TEFC enclosure is closed to atmosphere and relies on an external fan to
force air around the outside of the motor housing. Cooling is not as efficient
as the ODP enclosure and often, one finds that the motor service factor is
reduced when these enclosures are employed (check out the “Puzzler” for
more about service factor). A variant of the TEFC enclosure is known as
totally enclosed non ventilated (TENV). These enclosures are normally used
in direct drive fan applications and rely on the air moved by the driven
machine for their cooling. In non-fan applications, they may be fitted with a
water jacket (heat exchanger) for cooling.
The submersible sewage pump motor also uses a TENV enclosure that has
been modified in such a way as to prohibit water from entering the motor
housing. Instead of a fan, the surrounding water is used to transfer heat
from the motor housing. In unsubmerged applications, a water jacket may
also be employed.
TO OIL OR NOT TO OIL
So far so good. We now know that the
submersible sewage pump motor is just an
under water version of its TENV cousin. With
this in mind, let the arguments begin. The
figure to the left is a cross section of a
typical submersible sewage pump. Although
designs vary by manufacturer, each have a
motor housing and rotor that are close
coupled to a non-clog pump end. Higher end
pumps will incorporate a dual shaft seals with
an oil filled chamber between. Some
manufacturers will install a seal leak sensor in
this chamber to detect outer seal leakage
before water has the opportunity to enter the motor housing. Some will also
use a multiple sealing arrangement at the cord entry that includes epoxy
potting of the individual motor leads. This step eliminates the “wicking” of
water into the motor housing if the cord insulation is damaged. Sealing is a
very important topic and probably has a greater impact on pump life than the
motor fill. But our discussion today centers around why some manufacturers
fill these motor housings with oil while others do not.
THE PROS & CONS
Unfortunately the stated pluses and minuses of oil and air filled motors are
not always objective. If you check the literature you probably will not find
an objective comparison by a knowledgeable, independent author. Most of
our own prejudices are influenced by the manufacturer’s themselves. And,
why should we blame them? After all, their objective is to sell their own
products, not those of their competitors. Lets see if we can remedy this
situation by taking an unbiased look at each point of contention and either
sustain it or cast it aside. And, by the way, I will let you make the final
judgment.
The tables below list some of the common pros and cons that are tossed
about by the competing manufacturer’s. You will notice that the pros of one
are, more often than not, the cons of the other
Air Filled Motors
Pros: Higher motor efficiency
Better bearing lubrication
Longer life
Easier maintenance
Cons: Poor heat transfer
More frequent maintenance
Oil Filled Motors
Pros: Better heat transfer
Better bearing lubrication
Longer life
Permanent Lubrication
Cons: Lower efficiency
Environmental hazard
More difficult to maintain
Efficiency
The efficiency of a three phase induction motor is dependent upon the
quality of its materials, how well they are assembled, and the design of the
rotor / stator assembly. If we were to look at the published efficiencies of
three phase, horizontal T-Frame motors produced by various main stream
manufacturers, we would find that they vary by no more than a few tenths
of a percent. Also, we would find that larger motors are more efficient that
smaller ones. Typical efficiencies for T-Frame motors range from about 84%
- 90% for motors under 10HP and increase to 96% as horsepower
approaches 200. Single phase motors are much less efficient and tend to lag
three phase units by 7% - 15%.
Submersible sewage pump motors are similar in design to T-Frame motors
but tend to utilize a more elongated stator in order to reduce the motor’s
overall diameter. This design trade off results in a lower maximum
efficiency when compared with a T-Frame motor of the same horsepower
and speed. This efficiency reduction is especially evident in submersible
turbine pump motors where their diameter must be greatly reduced in order
to allow installation in a narrow well casing.
Oil filled motors are often accused of being less efficient because of a
purported increase in the clearance between the stator and rotor. This
increase, which is presumably required to accommodate the oil, is said to
reduce the stator’s ability to induce a charge on the rotor. Although this
may be the case in some models, all of the oil filled motors, with which I am
familiar, utilize the same rotor / stator clearance as air filled models. What
does reduce an oil filled motor’s efficiency is the energy loss due to oil
circulation within the stator housing. As a rule of thumb, this circulation
requirement reduces efficiency by about 1.5% when compared to an air filled
motor.
Although 1.5% may sound significant, we have to remember that motor
efficiency is not our only concern. Pump efficiency is an equally important
factor and, typically, pumps are not nearly as efficient as motors. In fact
the efficiency of the total machine (pump and motor), known as wire to
water efficiency, is the product of the two individual efficiencies. For
example, when a pump with a hydraulic efficiency of 70% is connected to a
motor that is 90% efficient, the efficiency of the entire machine is only
63%. If motor efficiency is reduced to 88.5% the total efficiency drops
just 1% to 62% - - so efficiency does not appear to be a major issue.
Heat Transfer
In an ODP motor, heat transfer is accomplished through conduction.
Although some conduction occurs via direct contact of the stator laminations
with the stator housing, the majority is due to air flowing around and
through the stator. Normally we think of convection as the heat transfer
agent when a fluid such as air is involved but, if that fluid is flowing the
physics changes and the result, although complex, is a combination of both
conduction and convection.
In a TENV motor, the spinning rotor causes air movement within the stator
housing and both conduction and convection are involved. In this case most
of the conduction occurs through stator / housing contact, but some is also
due to moving air contacting the motor housing. The major difference, as
compared to the ODP motor, is that there is a finite amount of air within
the motor and therefore, its ability to transfer heat to the housing is
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limited. Once heat is transferred to the motor housing, it is removed by
forced air or a water jacket.
Air filled submersible pump motors cool themselves in exactly the same way
as the TENV motor. They do have a distinct advantage in that water is in
direct contact with the motor housing and conducts heat more rapidly than
does the air provided by a fan or even an external water jacket. But, if it is
operating unsubmerged that advantage disappears.
The oil filled motor also cools itself primarily through stator / housing
conduction, however, it does have an additional advantage. The circulating oil
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This limited air volume can also contribute to hot spots in the stator windings of the TENV
motor that can reduce stator life.
with its greater heat capacity and thermal conductivity transfers more heat,
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more quickly to the motor housing than does air. For this reason, oil filled
motors tend to operate at somewhat lower temperature than do air filled
models. Oil filled motors also reduce the possibility of winding hot spots by
providing a more even heat distribution. This is a particular advantage when
Class B winding insulation is employed. For example, even a small increase in
operating temperature from 100ºC to 110ºC reduces the projected life of
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Class B insulation from 140,000 to 80,000 hours. With Class H insulation
this advantage is less significant at operating temperatures below 120ºC.
Another factor that affects operating temperature is the stator mounting
method. If the stator is pressed into the motor housing the metal to metal
surface area contact becomes greater when compared to a non-pressed
(bolted) stator. The result is better conductive heat transfer and, this is
the reason non-submersible, T-Frame motors utilize pressed in stators. But,
there is also a significant advantage to the bolted stator - - it is easier to
remove and replace and therefore reduces the cost of maintenance. When a
submersible motor is filled with oil, the conductive differences between the
two stator installation methods becomes moot because the oil’s greater
thermal conductivity increases the heat transfer between the stator and
the housing.
Both oil and air filled motors can operate in an unsubmerged environment.
Although oil filled motors transfer heat more rapidly to the motor housing
than do air filled models, how long either can operate depends more upon the
mass of the motor than it does with the fill fluid. After all, one could easily
design a TENV motor of almost any horsepower that can cool itself without
outside intervention if the size of the motor and its cost were not a concern.
For example, if one were to construct a 2 HP submersible motor on a 10 HP
frame, it could run unsubmerged indefinitely. Its sheer mass and surface
area would provide the necessary cooling. Unfortunately, one does not often
have that luxury when designing a submersible sewage pump. Therefore
supplemental cooling is usually required. In some instances diversion of a
portion of the pumpage so that it flows over the motor body is sufficient. In
other cases, a cooling jacket and an external liquid source is required.
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Conductivity @ 200ºF Air k=0.0181 Shellflex 210 Oil k= 0.0741
Lubrication
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Per IEEE 117 & 101
Depending upon the application, a motor’s bearings must support both axial
(back & forth) and radial (sideways) forces. For example, when a T-Frame
motor is flex coupled to a base mounted centrifugal pump, the motor’s
bearings will see radial forces generated by the spinning rotor and the
torque generated by the pump. They experience very little, if any, axial
loading. That same motor in a belt drive application would undergo an
additional radial force generated by the tension on the belt.
In the case of close coupled pump motors, where the impeller is attached
directly to the motor shaft, these same radial forces exist but are joined by
axial forces. The axial forces are either backward or forward loads that are
generated by the pump and its application environment. Vertical close
coupled pumps, like the submersible sewage pump, see an additional axial
force in the form of the hanging weight of the rotor/impeller assembly.
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Roller and ball bearings belong to a somewhat misnamed family known as
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antifriction bearings. These bearings reduce friction and wear by utilizing a
thin film of oil or grease to separate the balls or rollers from their inner and
outer races. The sole purpose of this lubricating film is to reduce metal to
metal contact. Theoretically, this film could be only a single molecule thick
and still meet its objective.
Although there are ongoing debates over which form of lubrication, oil or
grease, provides superior protection, it appears that both perform equally
well in most applications. Under conditions of extremely high heat grease is
preferred, while oil is the choice in unusually high thrust applications. Since
the submersible sewage pump motor typically sees neither of these
extremes, either should suffice.
The air filled motor’s bearings are usually grease lubricated. An exception is
the high thrust vertical hollowshaft motor where the lower bearing resides
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An extreme example of an axial force due to hanging weight is the lineshaft turbine. Its
motor bearings not only carry the weight of the rotor and impeller(s) but also the weight of
hundreds of feet of shafting.
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Antifriction is usually defined as without friction which we know cannot be true. From the
physics viewpoint, antifriction is defined as against or opposed to friction. Since friction is
a true force, antifriction implies that the bearing must produce some force to oppose and
neutralize it. This is also an improbability at best!
in an oil chamber. Smaller air filled motors usually incorporate sealed
bearings. This type of bearing is greased for life, where life is defined as a
certain number of hours under certain conditions. Medium and larger motors
use regreaseable bearings. Oil filled motors, on the other hand, rely upon
their dielectric oil for lubrication. As long as the proper grade of lubricant
is used and the manufacturer’s instructions are followed, both lubrication
methods will perform satisfactorily.
One precaution, however, is advised. Over greasing of the bearings of air
filled motors, especially those with external grease fittings, must be
avoided. Grease can damage the stator winding insulation and create hot
spots by limiting heat transfer.
Maintenance and Serviceability
Most of the normal maintenance a submersible sewage pump receives tends
to be the same regardless of its fluid fill. Replacement of the power cord,
control cords, impeller, wear ring, and seals are very similar regardless of
the design. One area that can differ is relubrication. The oil filled motor
does not require relubrication as long as an inspection of the oil verifies that
it is has not been contaminated by water or degraded by excessive heating.
The air filled motor does require regreasing, but if is fitted with external
grease fittings this can usually be accomplished during normal maintenance
inspections. If external fittings are not supplied, the motor must be
disassembled for regreasing.
Although bearings are more reliable than ever and Class H insulation is
prolonging stator life, there will come a time when a motor must be
disassembled. With the oil filled motor, an extra step is required. The oil
must be drained and disposed of properly. If it was not damaged by
excessive heat (motor overload etc) it can be filtered and reused in seal
chambers or other white oil applications. Once the oil is removed; stator,
rotor, and bearing replacement proceed in the same manner as air filled
motors.
Pressed in stators, whether they reside in air or oil filled motors, require
that the stator housing be heated for removal and replacement. For this
reason, manufacturers of pressed stator motors offer replacement stators
preinstalled in the stator housing. Although the stator can be rewound, the
cost is often a wash. For this reason they are often replaced. Bolted
stators are less costly to rewind due to the reduced labor required for
removal and reinstallation.
Environmental Concerns
Finally, one of the concerns that is sometimes raised about oil filled motors
is the potential for the oil to enter the environment. Although modern
dielectric oils do not contain environmentally hazardous chemicals and are
not considered hazardous waste by the EPA, they are not readily
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biodegradable. Therefore, it is important to minimize the introduction of
any lubricant into our wastewater systems.
Catastrophic failures involving loss of oil from oil filled, double sealed pumps
are rare in monitored pump stations. Because of the very different
operating environments of the inner and outer seals, it is statistically
improbable that the inner seal will fail before the outer one. The vast
majority of the oil resides within the stator housing. The amount that
resides in the seal chamber is small and, in the event of an outer seal failure,
a seal leak probe located in the seal chamber detects the presence of water
and produces an alarm condition. This allows replacement of the outer seal
long before the possibility of an inner seal failure.
Most air filled motors also utilize a leak probe in the seal chamber to detect
an outer seal failure. Some, however, locate the probe in the motor chamber
- - a design that can compromise the stator windings if both the outer and
inner seals fail.
SUMMARY
Well, there you have it -- a comparison of the pros and cons and, in a format
I believe to be reasonably accurate and unbiased. There are, without
question, several differences but are these differences really advantages or
disadvantages? Below is a summary of each.
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The same dielectric oil used in submersible sewage pumps is also used in oil filled
submersible turbine pump motors in the domestic water industry.
1) There is a small difference in air and oil filled motor efficiency but it has
little effect on the overall efficiency of the system.
2) The oil filled motor does run cooler and distributes heat more evenly, but
class H insulation may make heat transfer less important at temperatures
under 120ºC. In bolted stator designs oil also increases conductive heat
transfer.
3) Oil filled motors may also have an advantage in unsubmerged applications
where a water jacket is not used but it disappears when one is installed.
4) Bearing lubrication does not seem to be an issue if the manufacturer’s
instructions are followed, but care must be exercised when regreasing air
filled motors via external grease fittings. Oil filled motors usually do not
require relubrication.
5) Although oil must be drained from an oil filled motor prior to its
disassembly, most of the normal maintenance procedures are similar for
both designs. One disadvantage is found in air filled motors without external
grease fittings. They must be disassembled for regreasing.
6) Although dielectric oil is not a hazardous material, it is not readily
biodegradable and should not be introduced into the environment. Dual seals
with a internal leak sensor located in the seal chamber will prevent oil
leakage from the motor housing.